We observed stable isotopes in precipitation and atmospheric water vapor over a humid subtropical rice paddy field in Tsukuba, Japan, from June 2013 to May 2014. We used observed isotope ratios, in combination with an isotope-enabled general circulation model (GCM; Isotopes-incorporated Global Spectral Model: IsoGSM) to improve our understanding of the impacts of moisture sources and transport on the variability of water vapor isotopes. The isotopic measurements of water vapor and precipitation suggested that vapor isotopes in the study area were controlled by not only air-rain isotopic exchange, but also other kinetic effects associated with land evapotranspiration and large scale atmospheric circulation at the seasonal time scale. The contribution of land evapotranspiration to local water vapor content (FET) was approximately 16.0±12.3% as an annual average, with a summer maximum of 20.5±12.9%. Our results show that large-scale atmospheric circulation is the primary control on the variability of near surface water vapor δD. An IsoGSM tagging simulation experiment demonstrated that the large temporal variation of surface water vapor isotopes can primarily be attributed to advection and mixing of moisture from different oceanic source regions.